Washing apparatus with UV exposure and first and second ultrasonic cleaning vessels

ABSTRACT

Dirt, particularly of inorganic matter, attached to a substrate, such as a glass substrate for liquid crystal devices, is effectively removed by irradiating the substrate with ultraviolet rays including 184.9 nm and 253.7 nm in an oxygen-containing atmosphere in advance of wet cleaning with pure water. As a result, the wet cleaning time and the amount of pure water can be reduced.

This application is a division of application Ser. No. 08/743,375 filedNov. 4, 1996, (now U.S. Pat. No. 6,217,665), which in turn is acontinuation of application Ser. No. 08/013,314, filed Feb. 4, 1993 (nowabandoned).

BACKGROUND OF THE INVENTION

The present invention relates to a method of cleaning substrates,particularly a method of cleaning substrates suitable for cleaning glasssubstrates for liquid crystal devices wherein dirt on a glass substratecausing an inferior product is removed during a liquid crystal deviceproduction process.

There have been known wet cleaning techniques using pure water forcleaning substrates for precision devices or appliances, such as glasssubstrates for liquid crystal devices. In the case of cleaning a glasssubstrate for a liquid crystal device already provided with a pattern ofelectrodes and before provision of an alignment film, for example, ithas been ordinarily practiced to first remove dirt, such as dust andinorganic matter, by a combination of spraying, high pressure showeringand/or ultrasonic cleaning respectively using pure water, optionallywith brushing or ultrasonic cleaning with a detergent and cationic purewater as a pretreatment, and drain the water as by an air knife, aspinner or pulling out from warm pure water, or dry the substrate with,e.g., vapor of IPA (isopropyl alcohol).

It is also known to thereafter heat the glass substrate to about 150° C.and irradiate the substrate with ultraviolet rays at wavelengths of184.9 nm and 253.7 nm so as to have oxygen in the air absorb theultraviolet rays at 184.9 nm to generate ozone and have the ozone absorbthe ultraviolet rays at 253.7 nm to generate oxygen radicals, by whichorganic matter is decomposed and removed.

However, the above-mentioned first washing with pure water for removalof dust or inorganic matter with pure water as by a combination ofspraying, high pressure showering, ultrasonic cleaning, etc., requiressome length of time, thus leading to an inferior throughput of thecleaning apparatus or requiring an elongated apparatus in order toretain a high throughput using the same length of time. Further, a largeamount of water is required per sheet of glass substrate, and thecleaning cost is considerably expensive.

Further, if the pre-cleaning by brushing or ultrasonic cleaning using adetergent is performed before the cleaning with pure water, it ispossible to obtain a sufficient cleaning effect even if a shorter timeis used for the cleaning with pure water. However, for an identicalthroughput, this additionally requires a cleaning step using a detergentand a rinsing step, so that the total length of the required cleaningapparatus is not substantially changed. Further, the required amount ofpure water is not substantially changed either because the rinsing stepafter the cleaning with a detergent requires an additional amount ofpure water, thus also requiring a high process cost.

SUMMARY OF THE INVENTION

In order to solve the above-mentioned problems, an object of the presentinvention is to provide a method of cleaning a substrate, whereby dirt,such as inorganic and organic matters, can be effectively removed whileshortening the wet cleaning time and reducing the amount of water used.

Another object of the present invention is to provide a method ofcleaning a substrate, whereby a glass substrate can be effectivelycleaned with a minimum amount of pure water and a short time with asimple apparatus arrangement and without complex process control.

According to the present invention, there is provided a method ofcleaning a substrate for removing dirt on the substrate, comprisingirradiating a substrate surface with ultraviolet rays includingwavelengths of 184.9 nm and 253.7 nm in an oxygen-containing atmosphere,and then subjecting the substrate to wet cleaning with pure water.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a batch-type cleaning apparatus for use ina method of cleaning a substrate according to the present invention.

FIG. 2 is an illustration of a sheet-by-sheet type cleaning apparatusfor use in a method of cleaning a substrate according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of the present invention, a glass substratefor a liquid crystal device may be cleaned for removal of dirt thereonby first irradiating a surface of the substrate with ultraviolet raysincluding wavelengths of 184.9 nm and 253.7 nm in an oxygen-containingatmosphere and then subjecting the substrate to wet cleaning with purewater.

More specifically, in the cleaning method for removing dirt (foreignmatter) on a glass substrate according to the present invention,immediately before the wet cleaning with pure water of the substrate,the surface of the substrate is simultaneously irradiated withultraviolet rays including components with wavelengths of 184.9 nm and253.7 nm, preferably having peaks at these wavelengths, whereby oxygenin the atmosphere absorbs ultraviolet rays at 184.9 nm to form ozone andthe ozone absorbs ultraviolet rays at 253.7 nm to form oxygen radicals,with which the dirt of organic matter on the glass substrate ischemically removed and the surface tension of the glass surface isreduced to improve the wettability in advance to enhance the effect ofcleaning dirt of inorganic matter in a subsequent cleaning step withpure water.

The irradiation means for issuing the above-mentioned wavelengths may beany, provided that they include sufficient amount of the above-mentionedwavelengths. Examples of which may include: discharge lamps, such as lowpressure mercury lamps, black light fluorescent lamps, fluorescentchemical lamps, mercury arc lamps, and xenon arc lamps, and excimerlasers, such as KrF and ArF excimer lasers.

Such irradiation means can be combined in plurality as desired. It isalso possible to use separate irradiation means for a wavelength of184.9 nm and a wavelength of 253.7 nm.

The irradiation intensity of the ultraviolet rays can be varieddepending on the degree of soiling or dirt on the substrate or desiredcleanliness of the substrate but may generally preferably be at least0.2 J/cm², more preferably at least 0.4 J/cm².

The irradiation of the substrate with ultraviolet radiation may beperformed in an oxygen-containing atmosphere, which may conveniently beatmospheric air or preferably be an oxygen atmosphere or an atmospherecontaining oxygen diluted with an inert gas, such as Ar or N₂ so as tofurther obviate unnecessary by-products due to irradiation withultraviolet rays.

The time after the ultraviolet irradiation until the cleaning with purewater may generally be at most 30 minutes, preferably at most 10minutes. In other words, the cleaning with pure water may preferably beperformed when the substrate surface shows a contact angle with water ofat most 10 degrees, preferably at most 5 degrees. This means that thecleaning with pure water is started while the substrate surface showsgood wettability with pure water.

Anyway, standing for a long time after the ultraviolet irradiationshould be avoided, since the effect of the ultraviolet irradiation islost.

Hereinbelow, the present invention will be described with reference toan embodiment shown in the drawings.

EXAMPLE 1

FIG. 1 is an illustration of an outline of a batch-type cleaningapparatus for use in a method of cleaning substrates for, e.g., liquidcrystal devices. Referring to FIG. 1, the apparatus includes anultraviolet ray irradiation unit 1, wherein 7 U-shaped low pressuremercury lamps of 110 watts (“UVU-110”, available from K. K. OakSeisakusho) having two peaks and wavelengths of 184.9 nm and 253.7 nmwere arranged. Into the unit 1, glass substrates 3 (300 mm×300 mm×1.1mm-t), each provided with a surface pattern of electrodes and held in acleaning cassette 4, were supplied sheet by sheet for irradiation withultraviolet rays for 30 seconds per sheet from a distance of about 10mm.

Then, by an automatic conveying machine, 5 sheets of the glasssubstrates 3 subjected to the ultraviolet irradiation together with thecleaning cassette 4 were dipped and washed for about 180 seconds in afirst ultrasonic cleaning vessel 5 using pure water, and then dipped andwashed for about 180 seconds in a second ultrasonic cleaning bath 6,followed by drying with IPA (isopropyl alcohol) vapor in a chamber 7-1in a drying vessel 7. The substrates thus cleaned were then taken outfrom the cleaning apparatus and subjected to coating with a polyimideforming liquid by flexograhic printing, whereby a clear polyimide filmwas found to be formed thereon. The first and second cleaning vessels 5and 6 were respectively supplied with 500 liters/hour of pure water and,as a result of simple calculation, the substrates were sufficientlycleaned with pure water in a small amount of about 8 liters/sheet.

In contrast thereto, the same level of cleaning required about 16liters/sheet without the preliminary ultraviolet irradiation prior tothe cleaning with pure water.

For evaluating the cleaning performance, glass substrates 3 wereintentionally soiled with silica latex particles with an averageparticle size of 1.2 μm at a rate of about 300 particles/mm² and thencleaned in the above-described manner, whereby an extremely good removalrate of 98% was obtained.

In contrast thereto, when substrates intentionally soiled similarly asabove were cleaned without being introduced into the ultraviolet rayirradiation unit 1, i.e., by directly introduced into the first cleaningbath 5, the second cleaning bath 6 and the drying bath 7, a removal rateof only 92% was obtained showing a clearly inferior cleaning state thanin the case where the ultraviolet irradiation was performed in advanceof the cleaning with pure water. Further, in order to obtain a removalrate of 98%, it was necessary to effect the cleaning sequence though thevessels 5-7 two cycles under identical conditions.

EXAMPLE 2

FIG. 2 is an illustration of a sheet-by- sheet cleaning apparatus forpracticing a cleaning method for liquid crystal device substrates.Referring to FIG. 2, the apparatus includes an ultraviolet rayirradiation unit 1 wherein 5 U-shaped 110 watt low pressure mercurylamps (“UVU-110”, available from K. K. Oak Seisakusho) having two peaksat wavelengths of 184.9 nm and 253.7 nm were arranged. Through the unit1, glass substrates 3 (300 mm×300 mm×1.1 mm-t), each provided with atransparent electrode film (ITO) on the entirety of one face, wereconveyed continuously sheet by sheet to be irradiated with ultravioletrays from a height of 10 mm for about 40 seconds.

Then, the substrates 3 were subjected to wet cleaning by being sprayedwith warm pure water at about 30° C. from a spray nozzle 9 and thensubjected to high pressure showering of pure water at about 15 kg.f/cm²from a shower nozzle 10, followed by draining with air knife 11. Thethus cleaned substrates were then satisfactorily coated with apositive-type photoresist by roller coating, followed by satisfactorypatterning of the ITO film.

For evaluating the cleaning performance similarly as in Example 1,substrates 3 were intentionally soiled with silica latex particles withan average particle size of 1.2 μm at a rate of about 300 particles/mm²and then cleaned in the above-described manner, whereby a good removalrate of 96% was obtained in the case where the ultraviolet irradiationwas performed before the cleaning with pure water. In contrast thereto,an inferior cleaning rate of about 89% was measured in the case wherethe wet cleaning alone was performed.

EXAMPLE 3

The substrates cleaned in Examples 1 and 2 were again subjected toirradiation with ultraviolet rays in an oxygen-containing atmosphereunder similar conditions as in the previous examples, whereby furthereffective cleaning of the substrate surfaces could be performed.

As described hereinabove, according to the present invention, asubstrate surface is irradiated with ultraviolet rays includingwavelengths at 184.9 nm and 253.7 nm in an oxygen-containing atmosphereimmediately before wet cleaning with pure water, whereby it becomespossible to increase the removal rate of dirt, particularly of inorganicmatter. As a result, it is possible to shorten the wet cleaning time anddecrease the amount of pure water, leading to a decrease in productioncost.

What is claimed is:
 1. An apparatus for washing glass substratescomprising: a cassette for holding a plurality of sheets of glasssubstrate, each glass substrate having thereon a transparent electrode,prior to irradiation with ultraviolet rays and for holding a pluralityof irradiated sheets of glass substrate after irradiation; ultravioletexposure means for irradiating said sheets of glass substrate withultraviolet rays; holding means for moving a sheet of glass substratefrom said cassette to said ultraviolet exposure means for irradiationand for moving an irradiated sheet of glass substrate from saidultraviolet exposure means back to said cassette; ultrasonic washingmeans consisting of a first ultrasonic cleaning vessel containing aliquid consisting of water for immersing the cassette to wash theplurality of irradiated sheets of glass substrate and a secondultrasonic cleaning vessel containing a liquid consisting of water forimmersing the cassette to wash the plurality of irradiated andonce-cleaned sheets of glass substrate, said second ultrasonic cleaningvessel disposed immediately adjacent to said first ultrasonic cleaningvessel; a drying vessel containing isopropyl alcohol for drying thereinthe plurality of sheets of glass substrate, said drying vessel disposedimmediately adjacent to said second ultrasonic cleaning vessel; andconveying means for moving said cassette holding said plurality ofirradiated sheets of glass substrate from said first ultrasonic cleaningvessel to said second ultrasonic cleaning vessel to said drying vesselin this order.
 2. The apparatus according to claim 1, wherein theultraviolet rays exhibit wavelength peaks at 184.9 nm and 253.7 nm.
 3. Aprocess for producing a liquid crystal device comprising selecting atleast one glass substrate having thereon a transparent electrode andwashing said glass substrate using an apparatus according to claim
 1. 4.A process for producing a liquid crystal device comprising selecting atleast one glass substrate having thereon a transparent electrode andwashing said glass substrate using an apparatus according to claim 2.